In efforts to prevent global warming amid strong demands for clean alternative energy, new technology is urgently needed that is capable of efficiently converting sunlight to secondary energy (electric power, hydrogen, and the like). Expectations are growing for light-to-secondary energy conversion elements (i.e., elements converting light to secondary energy), such as solar cells and hydrogen generating photocatalysts, that exhibit a high light-to-secondary energy conversion efficiency (i.e., efficiency with which light is converted to secondary energy). In energy conversion, typical solar cells, hydrogen generating photocatalysts, and like light-to-secondary energy conversion elements utilize only part of the broad spectrum of sunlight below a certain threshold wavelength that is unique to the individual light-to-secondary energy conversion elements, failing to utilize those components that have longer wavelengths than the threshold wavelength. Thus, photon upconversion, in which the wavelengths of light are converted by absorbing relatively long wavelengths of light and emitting relatively short wavelengths of light, is being studied as one of technologies for effectively utilizing the broad spectrum of sunlight.
Research on photon upconversion by means of multiphoton absorption by rare-earth elements has a history of more than 50 years. Rare-earth elements, however, generally need very high incident light intensity for multiphoton absorption, which makes it difficult to target weak light, such as sunlight, for conversion in this method.
Several publications have been made recently about organic molecules capable of photon upconversion by means of light absorption and emission.
Patent Document 1 describes compositions by which photon energy upconversion takes place that contain at least a first component (e.g., phthalocyanine, a metal porphyrin, or a metal phthalocyanine) and a second component (e.g., a polyfluorene, an oligofluorene, a copolymer of these compounds, or a polyparaphenylene). The first component acts as a photon receptor that absorbs energy in a first wavelength range. The second component acts as a photon emitter that emits energy in a second wavelength range.
Non-patent Document 1 describes photon upconverters that exploit triplet-triplet annihilation (hereinafter, “TTA”) in organic molecules for upconversion of sunlight or similar, relatively weak light in a toluene solvent.
Some existent photon upconverters contain a high molecular weight organic polymer as a medium for organic molecules (see Non-patent Documents 2 and 3).
Patent Document 2 describes a photon upconversion system made up of at least one polymer and at least one sensitizer containing at least one type of heavy atoms, where the sensitizer has a higher triplet energy level than the polymer.
Non-patent Document 2 describes a photon upconverter that uses a polymer of cellulose acetate (molecular weight: approximately 100,000) as a dispersion medium for organic molecules.
Non-patent Document 3 describes a photon upconverter that uses, as a medium, a rubbery polymer with a glass transition temperature (Tg) of 236 K (−37° C.) that is soft at room temperature.
Non-patent Document 4 describes a photon upconverter that uses an oligomer of styrene (mixture of a trimer and a tetramer of styrene) as a medium for organic photosensitizing molecules and organic light-emitting molecules.
Non-patent Document 5 describes: metal porphyrins as organic photosensitizing molecules that can be used in TTA photon upconversion; diphenylanthracene, 9,10-bis(phenylethinyl)anthracene, and 9,10-bis(phenylethinyl)naphthacene as organic light-emitting molecules; and toluene as a medium for the organic photosensitizing and light-emitting molecules.
Non-patent Document 6 describes: a boron-dipyrromethene (BODIPY) derivative as a sensitizer for TTA photon upconversion; perylene or 1-chloro-9,10-bis(phenylethinyl)anthracene as an acceptor; and toluene as a medium.
Non-patent Document 7 describes photon upconversion where biacetyl is used as a sensitizer, 2,5-diphenyloxazole (hereinafter, “PPO”) as light-emitting molecules, and benzene as a medium.
Non-patent Document 8 describes photon upconversion by means of polymethyl methacrylate film where 2-methoxy thioxanthone is used as a sensitizer and PPO as light-emitting molecules.
The TTA-based photon upconverter, in principle, requires that organic molecules diffuse and collide with each other in a medium for energy transfer. Most prior art (Non-patent Documents 1, 4, 5, 6, and 7) uses as a medium either a volatile organic solvent, such as toluene or benzene, or a highly volatile medium, such as a styrene oligomer. These volatile organic solvents and highly volatile media (e.g., styrene oligomers), however, create safety issues due to their flammability. They also forbid use of resin materials that, when used in or as a container for an optical wavelength conversion element, may dissolve in the media or swell due to permeation of the media, which is inconvenient.
TTA-based photon upconverters that use a polymer compound, such as cellulose acetate or a soft rubber, as a medium (Patent Document 2 and Non-patent Documents 2, 3, and 8) have a problem that the upconversion emission intensity markedly decreases at room temperature (300 K) or below because the polymer compound is flammable and either solid or poorly fluidic at normal temperature (300 K). Non-patent Document 3 describes that the upconversion emission intensity is sufficiently high at relatively high temperatures (>300 K) where the polymer is sufficiently fluidic, but very low at low temperatures (≤300 K) where the medium is poorly fluidic because TTA photon upconversion requires that the organic molecules, responsible for producing triplet excitation energy, diffuse and collide with each other in a medium for energy transfer between the organic molecules.
To solve these issues/problems, the inventors of the present invention propose an optical wavelength conversion element for TTA photon upconversion produced by dissolving and/or dispersing organic photosensitizing molecules and organic light-emitting molecules in an ionic liquid. The proposed optical wavelength conversion element addresses conventional problems including the low upconversion emission intensity due to high viscosity of the medium, the flammability of the medium, and the volatility of the medium (Patent Document 3).
Patent Document 3, however, makes no specific proposals for an optical wavelength conversion element that converts ultraviolet-to-visible light to ultraviolet light.
Non-patent Documents 7 and 8 describe photon upconversion of visible light to ultraviolet light in the 350 nm to 440 nm region (this region is close to the visible region). No prior art, however, seems to disclose conversion to ultraviolet light with a local maximum emission wavelength of 360 nm or below.
There is hence a strong demand for optical wavelength conversion elements that exhibit a good temporal stability and a high optical wavelength conversion efficiency (upconversion emission intensity), that address conventional problems including the flammability and volatility of the medium, and that are capable of converting light in the ultraviolet-to-visible region to light in a shorter wavelength range (e.g., ultraviolet light).